1. Field of the Invention
The present invention relates to a semiconductor device having a guard ring.
2. Description of the Related Art
A semiconductor device has a plurality of circuit regions formed on a single semiconductor substrate. Therefore, noise generated from a single circuit region may be transmitted to another circuit region and influence the operation of a circuit therein. This influence is particularly serious in a system LSI which has both a digital circuit and an analog circuit for an analog circuit is particularly apt to be influenced by substrate noise.
For example, Japanese Patent Laid-Open Publication No. H9-326468/1997 and Japanese Patent Laid-Open Publication No. 2001-44277 disclose techniques of forming a guard ring between a circuit region where a circuit to be a noise source is formed (hereinafter called “noise source circuit region”) and a circuit region where a circuit susceptible to substrate noise is formed (hereinafter called “damageable circuit region” in order to restrain propagation of substrate noise. FIG. 1 is a plan view showing a conventional semiconductor device having a guard ring.
As shown in FIG. 1, internal circuit regions 2a and 2b are formed apart from each other on and above the top surface of the P type substrate 1. For instance, the internal circuit region 2a is a noise source circuit region and the internal circuit region 2b is a damageable circuit region. A P+ type diffusion region (not shown) is formed on the top surface of the P type substrate 1 in such a way as to surround the internal circuit region 2b and serves as a guard ring. A shunt wiring 44 is formed directly above the P+ type diffusion region and is connected to the P+ type diffusion region by a plurality of contacts (not shown). The shunt wiring 44 is connected to a ground potential wiring GND to be applied with the ground potential.
In the conventional semiconductor device shown in FIG. 1, the ground potential is applied to the guard ring, comprised of the P+ type diffusion region, via the ground potential wiring GND, the shunt wiring 44 and the contacts. As part of substrate noise generated from the internal circuit region 2a is absorbed by the P+ type diffusion region, the propagation of the substrate noise to the internal circuit region 2b is restrained.
The prior art however has the following problem. FIG. 2 is a graph showing the frequency dependency of the performance of the guard ring of the semiconductor device shown in FIG. 1, taking the frequency of substrate noise on the horizontal axis and taking on the vertical axis the intensity of substrate noise which propagates over the guard ring (P+ type diffusion region) to a damageable circuit region (internal circuit region 2b). As shown in FIG. 2, the frequency dependency is hardly seen in the performance of the guard ring. In case where a specific frequency component in substrate noise is strong, such as a case where the internal circuit region 2a (noise source circuit region) is a clock circuit region, therefore, this frequency component cannot be eliminated sufficiently. Depending on the type of a circuit to be formed in the damageable circuit region, the circuit may be particularly susceptible to a specific frequency component. The prior art cannot adequately protect the circuit in the damageable circuit region even in such a case.